At most cases Windows applications are very complicated. They put large number of traces to your system. Especially that refers to such things as entries in Windows Registry, Program Files and Shortcuts. In general the problem lies in Uninstaller (simply, an uninstaller is the opposite of an installer. During the uninstallation, the uninstaller removes all the components which were installed) that can't properly remove all traces in Windows. Also some users simply doesn't use uninstaller! They just delete the application program files from hard drive. Other traces still stays untouched! Large amount of junk traces will make your PC much slower.